1. Field of Invention
The present invention relates to determination of water saturation in subsurface formations surrounding a well borehole using radioactivity well logging.
2. Description of Prior Art
In evaluating reservoirs as possible candidates for enhanced recovery programs, it is important to determine residual oil saturation of the earth formations adjacent well boreholes. The accurate determination of water saturation in reservoir formations was a requisite to determining oil saturation for locating new reserves in old wells. A high degree of accuracy was particularly important when determining whether or not to commence expensive enhanced recovery projects. It was a desirable goal to determine oil saturation to within .+-.3 saturation units.
In the past, pulsed neutron sources have been used to bombard the formations and measure from the response of the formations the macroscopic thermal neutron capture cross section, .SIGMA., of the formation. When the formation porosity, shale fractional volume and thermal neutron capture cross-section of the shale and rock matrix were known, the thermal neutron capture cross-section of the fluid, .SIGMA..sub.f, could be determined. From the value of .SIGMA..sub.f, a measure of water saturation was obtained. However, errors in the values of the parameters used in determining water saturation limited the accuracy of this method.
Log-inject-log techniques have been developed to eliminate some of the parameters in determining residual oil saturation. These techniques involved successive logging after modification of the formation fluids by various injection methods. However, the accuracy of these techniques was still less than desirable.
Another method of determining water saturation is disclosed in U.S. Pat. Nos. 3,930,153 and 3,930,154, of which Applicant is inventor, which utilized capture gamma spectroscopy logging. A NaI (T1) gamma ray detector sensed gamma radiation from thermalized neutrons originally emitted by a pulsed neutron source. The gamma ray spectra of the formation were recorded and then analyzed for the presence of chlorine and hydrogen. The analysis was complex, requiring a least squares fit of the formation spectra to a postulated formation spectra.